Nervous system disorders affect millions of people, sometimes causing death and a degradation of life. Central and peripheral nervous system disorders include epilepsy, Parkinson's disease, essential tremor, dystonia, and multiple sclerosis (MS). Other nervous system disorders include mental health and psychiatric disorders, which also affect millions of individuals and include anxiety (such as general anxiety disorder, panic disorder, phobias, post traumatic stress disorder (PTSD), and obsessive compulsive disorder (OCD)), mood disorders (such as major depression, bipolar depression, and dysthymic disorder), sleep disorders (narcolepsy), obesity, and anorexia.
Epilepsy is the most prevalent serious neurological disease across all ages. Epilepsy is a group of neurological conditions in which a person has or is predisposed to recurrent seizures. A seizure is a clinical manifestation of neurological activity indicative of a nervous system disorder, and results from excessive, hypersynchronous, abnormal electrical or neuronal activity in the brain. This electrical excitability of the brain may be likened to an intermittent electrical overload that manifests with sudden, recurrent, and transient changes of mental function, sensations, perceptions, and/or involuntary body movement. Because seizures are unpredictable, epilepsy affects a person's employability, psychosocial life, and ability to perform otherwise standard tasks such as operating vehicles or heavy equipment.
Treatment therapies for epilepsy and other nervous system disorders can include any number of possible modalities alone or in combination including, for example, electrical stimulation, magnetic stimulation, drug infusion, and/or brain temperature control. Each of these treatment modalities can be operated using closed-loop feedback control. An exemplary closed-loop feedback control technique includes receiving from a monitoring element a neurological signal that carries information about a symptom, a condition, or a nervous system disorder. The neurological signal can include, for example, electrical signals (such as EEG, ECoG, and/or EKG), chemical signals, other biological signals (such as change in quantity of neurotransmitters), temperature signals, pressure signals (such as blood pressure, intracranial pressure or cardiac pressure), respiration signals, heart rate signals, pH-level signals, and peripheral nerve signals (cuff electrodes on a peripheral nerve). Monitoring elements include, for example, recording electrodes or various types of sensors.
Standard diagnostic EEG sensing requires two electrodes in contact with body tissue. The first electrode is placed near the desired source of the electrical activity that the physician desires to monitor, and is referred to as active. The second electrode, referred to as the reference, is typically placed outside of the cranium away from the desired source of electrical activity. For example, the reference electrode may be attached to the ear or mastoid, or at the back of the head. Such locations are considered “inactive” since sensing from these areas produces a potential that is close to zero. In a monitoring system or device, differential amplifiers measure the voltage difference between the reference electrode and other active electrodes located within the brain. The resulting intracranial signals are amplified and displayed as channels of EEG activity.
For implantable devices that perform EEG sensing, it is desirable to have the reference electrode contained within the body. The reference electrode is carefully positioned such that ECG and movement artifacts are not present in the measured signals. Also, if the signals are to be used for seizure detection, it is desirable that the reference electrode be remote from the seizure focus. The active electrodes are positioned either in direct or indirect contact with brain structures affecting a neurological condition for which sensing is being performed. For example, to treat epilepsy the active electrodes may be implanted in brain tissue at or near the seizure focus where they can sense EEG signals, detect a seizure event, and provide stimulation therapy. Conversely, the active electrodes may be positioned in an anatomical target distant from the seizure focus, but which is connected to the seizure focus by way of neuronal pathway projections. Activating pathway projections with electrical stimulation from a distant site (i.e., thalamus) may influence seizure activity at the focus (i.e., hippocampus/amygdala). With either approach, it is desirable to have a single electrode positioned away from the active electrodes, which can function as a reference for EEG sensing and/or function as an indifferent electrode for monopolar stimulation.
As previously alluded to, some devices that incorporate a feedback loop for sensing EEG signals also perform monopolar or bipolar tissue stimulation. Monopolar stimulation devices typically employ an implantable pulse generator, and a single lead having one or more active electrodes and a separate indifferent electrode. The active electrodes serve as the negative pole, and are normally disposed near the lead distal end. An indifferent electrode is frequently located on the exterior of the implantable pulse generator housing, which functions as the anode or positive pole. Electrical impulses occur as current flows between the active electrode and the indifferent electrode through the body tissue. Monopolar stimulation produces a radial current diffusion that covers an approximately spherical space around the active electrode.
In contrast to monopolar stimulation systems, bipolar stimulation systems utilize one or more electrodes as the positive pole, and one or more of the remaining electrodes act as the negative pole. The pulse generator housing is not used as an indifferent electrode. Usually, two adjacent or nearby electrodes are activated and respectively function as positive and negative poles. Bipolar stimulation creates a narrower and more focused current field than monopolar stimulation. However, monopolar stimulation is more frequently used because it usually requires lower stimulation parameters than bipolar stimulation to achieve the same clinical effect.
Despite the simplicity and effectiveness provided by a monopolar stimulation assembly, recent improvements in technology have created some situations in which the conventional coupling of the indifferent electrode to the implantable pulse generator housing is somewhat problematic. For instance, sometimes the implantable pulse generator can not function as a site for an indifferent electrode because the pulse generator already supports an electrode that is used for other functions such as recording electrocardiogram signals. In other cases, the pulse generator housing is made of a nonconductive material and can not support or function as an indifferent electrode. Also, in unusual circumstances patients have experienced sensations at the implantable pulse generator pocket during stimulation, perhaps attributable to the indifferent electrode.
Accordingly, it is desirable to provide systems and methods for performing EEG sensing and/or monopolar stimulation that overcome potential difficulties associated with implantable pulse generator size and function, and also reduce the potential for patient discomfort. In addition, it is desirable to provide methods and systems that can be adapted to accommodate a wide variety of implantable pulse generators, and to further accommodate the various methods for implanting and configuring such devices. Furthermore, other desirable features and characteristics of the present invention will become apparent from the subsequent detailed description and the appended claims, taken in conjunction with the accompanying drawings and the foregoing technical field and background. 